If the ItemSource property of a combo box has been set, why could clicking on the drop down botton of a combobox not display the list of available items? This may be related but within the same control, any context menu is displayed behind the user control:

first of all, have you tried adding a margin so you can see if it really is behind the rest of the UI or if it isn't drawn at all
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Markus HütterMar 5 '11 at 20:46

No, I have not done this. However, if I use a ListBox, I do see the list of available items that the combobox is not displaying. Given the related problem with context menus, I am suspecting combo box list is being drawn behind parent.
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Klaus NjiMar 5 '11 at 23:19

1 Answer
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There was a common WPF bug that caused this behavior of any Popup type UI displaying in back instead of topmost so that may be what you're seeing. I haven't seen the issue in a while but I'm not sure if it was fixed or not. It's related to bad video drivers so you might see it only on certain machines and not all the time.

There was a hotfix available from MS support but it might be easier to just switch your application to use software rendering instead.